There is an increasing need of delivering wireless technology with broadband capacity for cellular networks. A good broadband system must fulfil certain criteria, such as high data rate and capacity, low cost per bit, good Quality of Service and greater coverage. High Speed Packet Access (HSPA) is an example of a network access technology that enables this.
HSPA is a collection of protocols which improves the performance of existing Universal Mobile Telecommunication Systems (UMTS), which is a third generation (3G) cell phone technology. UMTS uses Wideband Code Division Multiple Access (WCDMA) as air interface for the radio-based communication between user equipment (UE), in form of a mobile terminal, and the base station (BS). The air interface in the Open Systems Interconnection (OSI) model comprises layers 1 and 2 of the mobile communications system, establishing a point-to-point link between the UE and a radio access node (RAN).
HSPA is an integral part of WCDMA. Wide-area mobile coverage can be provided with HSPA. It does not need any additional spectrum or carriers. Currently, WCDMA can provide simultaneous voice and data services to users on the same carrier. This also applies to HSPA which means that spectrum can be used efficiently. Simulations show that in a moderately loaded system, HSPA can largely reduce the time it takes to download and to upload large files. The primary benefits of HSPA are improved end-user experience. In practice, this means shorter UL and DL times as a result of higher bit-rates and reduced latency compared to earlier releases of WCDMA. HSPA also benefits operators by reducing the production cost per bit. More users can be served with higher bit-rates at lower production costs.
HSPA is the set of technologies defining the migration path of WCDMA operators worldwide. The two existing features, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), in the HSPA family provides the increased performance by using improved modulation schemes and by refining the protocols by which handsets and base stations communicate. These improvements lead to the better utilization of the existing radio bandwidth provided by UMTS.
High Speed Downlink Packet Access (HSDPA) is the first feature within HSPA. It is part of the WCDMA Third Generation Partnership Project (3GPP) Release 5 specification. HSDPA provides a new downlink transport channel that enhances support for high-performance packet data applications. It represents the first step in the evolution of WCDMA performance. HSDPA can deliver an up to 35 fold increase in downlink data rates of standard WCDMA networks, enabling users to access the Internet on mobile phones and laptops, at speeds previously associated with fixed line DSL.
HSDPA is based on shared channel transmission, which means that some channel codes and the transmission power in a cell are seen as a common resource that is dynamically shared between users in the time and code domains for a more efficient use of available codes and power resources in WCDMA. The radio channel conditions experienced by different downlink communication links vary significantly, both in time and between different positions in the cell. To compensate for rapidly varying radio conditions in the downlink, HSDPA relies on bit-rate adjustment. That is, while keeping transmission power constant, it adjusts (by lowering) the data rate by adjusting the modulation.
Along with the HS-DSCH (High Speed Downlink Shared Channel) physical channel on which payload data is sent, three new physical channels are also introduced: HS-SCCH, HS-DPCCH and HS-PDSCH. The High Speed-Shared Control Channel (HS-SCCH) informs the user that data will be sent on the HS-DSCH 2 slots ahead. The Uplink High Speed-Dedicated Physical Control Channel (HS-DPCCH) carries acknowledgment information and current channel quality indicator (CQI) of the user. This value is then used by the base station to calculate how much data to send to the user devices on the next transmission. The High Speed-Physical Downlink Shared Channel (HS-PDSCH) is the channel mapped to the above HS-DSCH transport channel that carries actual user data.
High Speed Uplink Packet Access (HSUPA) is the second feature within HSPA. It is part of the WCDMA Third Generation Partnership Project (3GPP) Release 6 specification. HSUPA provides a new uplink (UL) transport channel called Enhanced Dedicated CHannel (E-DCH). HSUPA dramatically increases the uplink data traffic rate. It provides a possibility to significantly increase the amount of data uploaded over mobile networks, especially user-generated content. Although a lot of it is downlink oriented, there are still quite a number of applications that will benefit from an improved uplink. These include the sending of large e-mail attachments, pictures, video clips, blogs etc. HSUPA is also known as Enhanced UL. In contrast to HSDPA, the new uplink channel that is introduced for Enhanced Uplink is not shared between users, but is dedicated to a single user.
FIG. 1 shows a HSUPA network overview. A user terminal 15 communicates with the core network CN via at least one base station 11. The system further comprises a second base station 10 with a corresponding system. A first radio network controller RNC 12 establishes an E-DCH which enables uplink data traffic from the user terminal to the base station. The E-DCH carries data for at least one radio network bearer. The term “lu” in FIG. 1 represents the interface between RNC and core network. The term “lub” represents the interface between RNC and the radio bases station (RBS).
Several new physical channels are added to provide and support high-speed data transmission for the E-DCH. As shown in FIG. 1, two new code-multiplexed uplink channels are added:                E-DCH Dedicated Physical Data Channel (E-DPDCH)        E-DCH Dedicated Control Channel (E-DPCCH)        
E-DPDCH carries the payload data, and the E-DPCCH carries the control information associated to the E-DPDCH. E-DPDCH is used to carry the E-DCH transport channel. There may be zero, one or several E-DPDCH on each radio link wherein there is at most one E-DPCCH on each radio link. E-DPDCH and E-DPCCH are always transmitted simultaneously. E-DPCCH shall not be transmitted in a slot unless E-DPDCH is also transmitted in the same slot.
Similarly, three new channels, see FIG. 1, are added to the downlink for control purposes:                E-DCH Hybrid Automatic Repeat Request (HARQ) Indicator Channel (E-HICH) carrying the uplink E-DCH hybrid Acknowledgement (ACK) and Negative ACK (NACK) indicator.        E-DCH Absolute Channel (E-AGCH) carrying absolute grants, which means that it provides an absolute limitation of the maximum amount of uplink resources the UE may use.        E-DCH Relative Grant Channel (E-RGCH) carrying the uplink E-DCH relative grants, which means that it controls the resource limitations by increasing or decreasing the limitations with respect to the current serving grant.        
E-AGCH is only transmitted from the serving cell. E-RGCH and E-HICH are transmitted from radio links that are part of the serving radio link set and from non-serving radio links.
As shown in FIG. 1 the same E-DCH can be provided both through the first RNC 12 for the serving cell and through a second RNC (RNC2) 13 for the non-serving cell. The second RNC 13 serves a separate base station 10 with a Node B NB2 and an enhanced UL scheduler (EUL-S2). Except for E-AGCH (which can only be transmitted through the serving cell) all the physical channels can be transmitted through either of the cells. As an alternative one RNC can serve both a serving cell and a non-serving cell. The term “lur” in FIG. 1 represents the interface between the first RNC 12 and the second RNC 13. Only one RNC will communicate with the core network (e.g. the first RNC). The first RNC is in control of the connection and handles things like soft handover.
Note that HSUPA channels are added on top of uplink/downlink dedicated channels. Each UE 15 therefore additionally carries an uplink and downlink dedicated physical channel (DPCH), see FIG. 1. In the downlink, a fractional dedicated channel (F-DPCH) can be used alternatively. The F-DPCH carries control information and is a special case of downlink Dedicated Physical Control Channel (DPCCH). UL might only contain the DPCCH as in FIG. 1. It could also contain a Dedicated Physical Data Channel (DPDCH). The F-DPCH has been introduced in 3GPP release 6 in order to optimize the downlink codes usage.
The UL scheduling is of central importance for HSUPA. It is provided by an enhanced UL scheduler (EUL-S) located in the Node B, see FIG. 1, close to the air interface. The task of EUL-S is to control the UL resources the UEs 15 in the cell are using. It operates on a request-grant principle where the UE requests a permission to send data and the scheduler decides when and how much data an UE is allowed to send and also how many UEs will be allowed to do so. With the EUL-S a scheme is introduced where the Node B controls the UL transmissions by providing grants for the UE. The cell appointed as serving cell (server by Node B NB) is the primary control of the scheduling mechanism by means of sending either absolute or relative grants. Thereby the maximum allowed HSUPA transmission is controlled. This effectively limits the transport block size the UE can select and thus the uplink data traffic rate. It enables the system to admit a larger number of high-data rate users and rapidly adapts to interference variations—leading to an increase both in capacity and the likelihood that a user will experience high data rates.
The grants are expressed as power headroom 14, for the E-DPDCH (grant) relative to the DPCCH transmission power (DPCCH (set)), that the UE 15 may use for scheduled transmissions. This is illustrated in FIG. 2. See also 3GPP 25.214, which for instance describes the relation between the E-DPDCH power and the DPCCH power value. In general, the power headroom defines the maximum allowed power offset, for instance for E-DPDCH.
Power (watt) is on the Y-axis and time (seconds) on the X-axis, FIG. 2. The DPCCH (set) power varies in dependency on the operation by the UE 15 and Node B and follows a SIR target, which will be described in relation to the power control of the DPCCH, DPCCH is power controlled as specified in the third generation partnership project 3GPP document TS 25.214. The E-DPDCH power is converted to a scheduled bit rate by the UE.
As a basic principle of the uplink scheduling mechanism, the UE 15 maintains a serving grant which at least represents the maximum E-DPDCH power offset, the power headroom, which the UE may use in the next transmission. This is illustrated in FIG. 3. Power (watt) is on the Y-axis and time (seconds) on the X-axis. The line E-DPDCH (used) is the present E-DPDCH power offset (read load) used by the channel at a certain time T (seconds). The available uplink power offset determines the possible data rate.
The absolute grants (AG0, AG1) are used to initialize the scheduling process and provide absolute transmit power offset/bit rate (the power headroom) for the UE 15. It allows the Node B scheduler to directly adjust the granted bit rate of UEs under its control. The relative grants (RG0) are used for incremental up- or downgrades (by a predefined step) from the currently used power headroom (transmit power). The absolute grant is carried by the downlink physical channel E-AGCH and the relative grant is carried by the downlink physical channel E-RGCH.
The power control results in less interference and allows more users on the same carrier. Power control thus provides more capacity in the network. There is a fast closed loop power control for all the UL signals to avoid power imbalance between different UE 15 signals and to combat fast fading. The Node B for instance measures continuously a signal-to-interference ratio (SIR) of the DPCCH transmitted by the UE. SIR relates to the fact that a certain DPCCH power is needed in relation to the interference so that the system is able to decode a data packet. Measurement shall be performed on the DPCCH. SIR is the quotient between the average received modulated carrier power and the average received co-channel interference power, e.g. cross-talk from other transmitters than the useful signal.
This real time SIR measured is compared to a SIR target provided by the RNC 12. The Node B transmits a power control (TPC) command in a downlink to the UE 15 to increase or decrease the transmit DPCCH power level so that the real time SIR measured is controlled towards the SIR target. This is for instance described in GB 2336740. The basic step is +/−1 dB/slot and eventually 2 dB. With this power control, the signals from different UEs can be received with the required quality at changing conditions.
All other physical channels are related to the DPCCH by means of the configured power offsets. The configuration of the power offset for E-DPDCH depends on the amount of data presently transmitted UL. In general DPCCH forms the basis for the rapid power control (1500 Hz+/−1 dB at each occasion). Dependent on the amount of data transmitted momentary (for instance in one frame or subframe for E-DPDCH) a power offset is decided for instance for the E-DPDCH. This power offset can be signalled via the control signalling to the UE 15 but can also be calculated by the UE from an extrapolation or interpolation from a limited number of reference points.
For E-DPDCH the power relates to the power level of DPCCH by the power offset. This power offset varies and is reconfigured continuously by the UE 15 and the RBS simultaneously. The real time value for the E-DPDCH power is calculated on the basis of the DPCCH power by the power offset. The E-DPDCH power if defined as the DPCCH power+the power offset for E-DPDCH. This power offset depends on the amount of data that should be sent in a certain TTI (Transmission Time Interval).
There is a capacity trade-off for the SIR on the DPCCH. Moreover, if DPCCH is increased the other channels are also increased, e.g. power offset on E-DPDCH. If the channel estimation (number of components, relative joint relationship) is optimized it is also possible to perform a maximum ratio combining—MRC, which means that the data bits can be decoded with the lowest possible SIR. The quality of the channel estimate depends on the SIR for the control bits (pilot bits on DPCCH) on which the estimation is based. The consequence is that the requirement on SIR for the data bits can be reduced if SIR for the control bits is increased.
At high power offsets for the other physical UL channels, in this case E-DPDCH in particular, the drawbacks relating to the increase of SIR for DPCCH is low compared to the benefits relating to improved channel estimation. A reduction of e.g. E-DPDCH power (due to improved_channel estimation) from a high offset creates a relatively large benefit. A high SIR target for DPCCH is beneficial to enable an improved channel estimate (estimated on the DPCCH) which will lower the required SIR for the E-DPDCH and thus the E-DCH load is reduced.
Raising the SIR target for DPCCH results in an improved channel estimate, but since SIR is the quotient between the channel power and the average received co-channel interference power, a raised SIR increases the_DPCCH power.
This result in the problem that at low power for the E-DPDCH the drawbacks relating to raising the DPCCH (by raising the SIR target for the DPCCH) can become significant compared to the benefits relating to an improved channel estimation (improved performance for E-DPDCH. A reduction of e.g. E-DPDCH (due to channel estimation) creates a relatively small benefit. The extra load in DPCCH at low power (grants) can not be motivated by the reduced E-DCH load (due to the low power offset), since the E-DCH load is anyway rather low.
The relationship between the E-DCH load and the DPCCH load is illustrated in FIG. 4. Line C relates to high E-DCH load (high grant), line A to low E-DCH load (low grant) and B to DPCCH. Load is on the Y-axis and SIR on the X-axis. Here it can be seen that in point H, which relates to high grant, the extra load in DPCCH (due to higher SIR which improves the channel estimate) can be motivated because of the reduced E-DCH load. In point L, which relates to low grant, the extra load in DPDCH can not be motivated by the reduced E-DCH load. The reason is that the E-DCH load is anyway rather low.